The present invention relates to a method of and apparatus for testing whether a natural diamond has had a layer of synthetic diamond deposited thereon. This is of particular importance in testing whether the diamond is wholly natural or whether any part of it comprises CVD diamond material and also in locating such material if present.
Synthetic diamond material may be deposited on an uncut or part processed natural diamond which is then worked, for example, into a round brilliant cut. Alternatively, the synthetic diamond material coating may be deposited onto a fully fashioned brilliant stone after working of the stone. The thickness of the synthetic diamond material layer may be very thin (it could be in the range from 5 microns to 10 microns) but the present invention may also be used to detect thicker layers.
The value of a diamond is in part dependent upon its weight. Accordingly, synthetic diamond material may be deposited onto natural gem diamonds, before or after cutting of the diamond, to increase the weight of the finished product.
However, the value of a diamond also resides in its qualities of authenticity and uniqueness and in the fact that it is an entirely natural (ie mined) product. Thus, a diamond that has not been enlarged by deposition of synthetic diamond material has a value over a diamond which has.
Over the years, a number of methods of synthesising diamond material have been developed. One of these methods is the chemical vapour deposition (CVD) technique, which is a low pressure technique involving deposition of synthetic diamond (referred to as CVD diamond material in this specification) onto a substrate from a gas. CVD is the most likely way in which synthetic diamond will be deposited on a diamond, although alternative techniques such as physical vapour deposition have been proposed. A diamond artificially enlarged by deposition of CVD or similar diamond material is referred to in this specification as a "CVD/natural diamond doublet".
CVD diamond material may be deposited on a non-diamond or diamond substrate. In the latter case, the CVD diamond material can replicate the structure of the diamond substrate (referred to as "homoepitaxial growth"). The CVD/natural diamond doublet produced can be identical in appearance, density and other common physical properties to an entirely natural stone and there may be a problem in identifying such a CVD/natural diamond doublet.
A method of testing whether a diamond has had a layer of synthetic diamond deposited thereon is disclosed in British Patent Application No. 9401354.7 published as GB2286251A. A plurality of parts of the diamond are irradiated with radiation substantially of wavelength substantially in the range 230 nm to 320 nm and the transmission of the irradiating radiation by the diamond is observed.
The invention of GB 9401354.7 is based upon the observation that where different zones of a diamond show differences in their absorption of radiation substantially of wavelength substantially 230 nm to 320 nm, it may be concluded that the diamond in question has a layer of synthetic diamond deposited thereon. It is further observed that if all zones of a diamond strongly absorb radiation substantially of wavelength substantially 230 nm to 320 nm, the diamond may be classified as almost certainly a wholly natural diamond.
The intensity of radiation transmitted by the zones of the diamond may be investigated using an imaging apparatus or by placing the diamond in an integrating sphere. Preferably, an image of the diamond is formed against a dark or light background.
It is an object of the present invention to provide a method of and apparatus for testing whether a diamond has had a layer of synthetic diamond deposited thereon, in which relatively simple imaging apparatus is used and an expensive integrating sphere is not required.
It is desired that the apparatus should be simple and inexpensive and may be put into operation by a person with relatively little training. The method and apparatus should be capable of being operated reliably and consistently by a practised jeweller who has no training in laboratory gemological analysis.